Include event details in listener response

NEW: The event listener now also returns the event details. Example:

webmonetization.set_listener(function(self, event, details)
    pprint(details)
end)

Details table:

{
  paymentPointer = "$ilp.uphold.com/QkG86UgXzKq8",
  assetScale = 9,
  amount = "26009",
  requestId = "a1f728aa-21e0-4376-ae99-0ccb22642956",
  assetCode = "XRP"
}